Method for producing a dispersible, fine titanium pyrophosphate powder

ABSTRACT

A novel method for forming a dispersable, fine titanium pyrophosphate powder by reacting titanium dioxide and a molar excess of phosphoric acid. The excess phosphoric acid, at a phosphorus pentoxide to titanium dioxide mole ratio greater than one, significantly improves the yield of the reaction product as a fine powder. A phosphorus pentoxide to titanium dioxide mole ratio of about 1.20 to about 1.25 results in over a 90% yield of a dispersable, fine titanium pyrophosphate powder having a particle size less than one micron in diameter.

This is a divisional application of U.S. patent application Ser. No.08/597,079 filed Feb. 5, 1996 now U.S. Pat. No. 5,733,519 issued Mar.31, 1998.

BACKGROUND OF THE INVENTION

This invention relates to a method for producing titanium pyrophosphate.More particularly, this invention relates to a method for thermallyreacting titanium dioxide and with an excess of phosphoric acid to forma dispersible, fine titanium pyrophosphate powder.

SUMMARY OF RELATED ART

Titanium pyrophosphate is a white crystalline material suitable for useas a UV-reflective pigment in paints to provide a protective coveringagainst radiation. The titanium pyrophosphate reflects ultravioletradiation and thereby reduces the absorption of higher energy solarradiation by the structure or object protected with the material. Thus,the heating and degradation of the structure or object are minimized bythe reflection of the ultraviolet radiation.

The ability of a pigment to reflect or scatter light is dependent on theparticle size, the refractive index, and the absence of lightabsorption. The average particle size of most pigments is about 0.01-1micron in diameter. The most effective pigment particle size for lightreflection or scattering is approximately half the wavelength of thelight. The average particle size of most white pigments, such astitanium dioxide, is in the range of 0.2-0.3 microns. Titanium dioxidepigments reflect visible light at 400-700 nm. For near ultravioletlight, 300-400 nm wavelength, pigment particle diameters near 0.1 to 0.2micron would be expected to be the most effective for reflection.Satisfactory reflectivity for titanium pyrophosphate is achieved if theparticles are in the size range 0.1 to 2.0 microns in diameter.

Reflectivity also depends on the difference between the refractiveindices of the pigment and the medium in which they are dispersed. Thegreater the difference, the higher the reflectivity. Additionally, theabsence of light absorption impacts the ability of a pigment to reflectlight. Although titanium dioxide is a highly reflective pigment in thevisible region of the spectrum, it absorbs and does not reflect light inthe near ultraviolet region. In contrast, titanium pyrophosphate doesnot possess an absorption band in the near ultraviolet region andretains its ability to reflect light.

Titanium pyrophosphate is generally produced by reacting a titaniumcompound with a phosphorus compound followed by heating to temperaturesin excess of 350° C. One conventional method of producing a titaniumpyrophosphate involves the formation of a titanium orthophosphateintermediate by reacting a titanium tetrahalide, such as titaniumtetrachloride, with phosphoric acid. The reactants are simultaneouslyintroduced in a volume of water to precipitate the intermediateorthophosphate. The orthophosphate compound is recovered from solutionand washed prior to forming the pyrophosphate. The orthophosphatematerial is calcined at temperatures of 750° C. to 1000° C. to formtitanium pyrophosphate.

Titanium pyrophosphate is also conventionally produced by thesimultaneous oxidation of titanium tetrachloride and phosphorusoxychloride at temperatures of about 1000° C. to 1100° C. The reactionresults in the formation of titanium pyrophosphate and chlorine as aby-product.

U.S. Pat. No. 3,996,332 discloses another process for producing titaniumpyrophosphate wherein titanium dioxide is mixed in a reaction vesselwith phosphoric acid at a phosphorus pentoxide to titanium dioxide moleratio (P₂ O₅ :TiO₂) of one to one. The reaction takes place in thereaction vessel at temperatures of 300-500° C. for one to twelve hours.The resulting product is utilized as a flux in the synthesis of rutilefrom titaniferous slag. The titanium pyrophosphate processes usingreaction components of titanium dioxide and phosphoric acid utilize theone to one phosphorus pentoxide to titanium dioxide mole ratio. Ingeneral, the resulting product from this particular process has agreater particle size than the desired fine powder. Furthermore, thegranular material resists disintegration, thereby making it difficult tomill the material to the desired particle size for use as an ultravioletreflecting agent.

Thus, existing titanium pyrophosphate processes involve either theformation of an intermediate, the reaction or calcination of compoundsat high temperatures, the formation of undesirable by-products, or theformation of titanium pyrophosphate particles having a greater particlesize than the desired fine powder.

It would be advantageous to produce fine titanium pyrophosphate powderwith a significantly high fine powder yield. Fine titanium pyrophosphatepowder is advantageously employed for use as a pigment or an opacifyingagent over coarse material because of the improved texture andreflective properties it imparts to the objects on which it is applied.

Furthermore, it would be advantageous to produce a dispersible, finetitanium pyrophosphate powder directly from a reaction mixture oftitanium dioxide and phosphoric acid. The reaction of titanium dioxideand phosphoric acid would eliminate the production of an intermediateorthophosphate or the production of undesirable by-products. Anintermediate orthophosphate requires additional processing steps,including high temperature calcination. Furthermore, undesirableby-products often result in additional processing or disposal costs. Theproduction of small particle size or fine titanium pyrophosphatedirectly from a reaction mixture would eliminate additional processingsteps and thereby reduce overall production costs.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method forproducing dispersible, fine titanium pyrophosphate powder by thermallyreacting titanium dioxide with phosphoric acid in a reaction mixturehaving an excess of phosphoric acid. The excess phosphoric acid in thereaction mixture results in a fine titanium pyrophosphate powder at asignificantly improved, and desirable, fines yield.

The method of the present invention generally involves the mixture oftitanium dioxide and hot concentrated phosphoric acid in a reactionvessel. The reaction mixture has a phosphorus pentoxide to titaniumdioxide mole ratio greater than one. Preferably, the phosphoruspentoxide to titanium dioxide mole ratio is about 1.06 or more, and mostpreferably about 1.20 to about 1.25. The reactants are thoroughly mixedand heated at temperatures of about 400° C. to about 500° C. to producea fine titanium pyrophosphate powder having a significantly higher finesyield over conventional titanium pyrophosphate processes. The finesyield is greater than 90% when using a phosphorus pentoxide to titaniumdioxide mole ratio of about 1.20 to about 1.25.

It is an object of the present invention to provide a method ofproducing a titanium pyrophosphate from titanium dioxide and phosphoricacid wherein the process results in a finished product exhibiting asignificantly higher fines yield than the existing processes known inthe art. The method of the present invention is carried out with anexcess of phosphoric acid to increase the amount of fines in theresulting titanium pyrophosphate product. Fine titanium pyrophosphatepowder is a desirable particle size for use as pigments and opacifyingagents in paints and organic polymers.

It is also an object of the present invention to produce a fine titaniumpyrophosphate powder directly from titanium dioxide and phosphoric acid.A method of producing a fine titanium pyrophosphate powder directly fromthe titanium dioxide and phosphoric reaction components eliminates thehigh temperature calcination of orthophosphate intermediates and theproduction of undesirable by-products.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, it has surprisingly beendiscovered that a significant improvement in the fines yield of titaniumpyrophosphate may be obtained from the reaction components of titaniumdioxide and phosphoric acid, wherein an excess of phosphoric acid isutilized in the reaction mixture. The resulting fine titaniumpyrophosphate powder is desirable for direct use as a UV-reflectivepigment.

The method of the present invention involves the direct reaction oftitanium dioxide with phosphoric acid. The reaction is generally carriedout in a reaction vessel at a temperature range of about 400° C. toabout 500° C. The reaction proceeds rapidly and may take place in eithera batch or continuous process. The reaction vessel dictates the amountof time required to fully react the titanium dioxide and phosphoricacid. The thorough and rapid mixing of the reaction components willlower the reaction time to several minutes while less thorough mixingmay require several hours to fully react the components in the mixture.

The reaction temperature of 400° C. to 500° C. may be maintained througheither the initial temperature of the phosphoric acid or through a heattransfer means on the reaction vessel. Highly concentrated phosphoricacid must be maintained at relatively high temperatures in order topermit fluid flow. Therefore, heat input from the reaction vessel maynot be required with high acid concentrations. However, if needed,sufficient heat to form the titanium pyrophosphate may be supplied byinductive heating, such as a hollow screw or in a furnace. Other heattransfer mechanisms are also suitable for maintaining the appropriatereaction temperatures.

The titanium dioxide utilized in practicing the present inventive methodmay include various commercially available forms. Either anhydrous orhydrated forms of titanium dioxide are suitable for use in the presentinvention. Anhydrous titanium dioxide is available in either the rutileor anatase forms. The titanium dioxide particle size utilized in thepresent invention is generally less than one micron in diameter.Typically, the average particle size is about 0.2 to about 0.3 micronsin diameter.

The phosphoric acid is a concentrated phosphoric acid having at least a75% H₃ PO₄ concentration. However, a highly concentrated phosphoric acidhaving an H₃ PO₄ concentration of at least about 115% is preferredbecause the highly concentrated forms of phosphoric acid are lesscorrosive than the lower acid concentrations and less water isevaporated during the process. The reaction temperatures of theinventive process, about 400° C. to 500° C., will concentrate the acidin the reaction mixture. The highly concentrated forms of the phosphoricacid are generally available from heat recovery units on phosphoric acidproduction processes.

The initial step in the method of the present invention is the thoroughmixing and heating of titanium dioxide with phosphoric acid. The mixingand heating may either be simultaneous or sequential. A batch orcontinuous reaction vessel may be utilized in practicing the presentinventive method. The process equipment should provide a means forthoroughly mixing phosphoric acid throughout the titanium dioxide.Incomplete mixing of the reaction components will result in theformation of an amorphous titanium rich phase. The amorphous phase willcause further agglomeration of the finished product.

An excess of phosphoric acid is utilized to obtain the desired finetitanium pyrophosphate power. A phosphorus pentoxide to titanium dioxidemole ratio greater than one is utilized to obtain the fine powder. Thefine powder is defined as particles having a particle size less than onemicron in diameter. The fines yield in the prior art processes istypically below 60%. The method of the present invention results in afines yield greater than 60%. Preferably, with a phosphorus pentoxide totitanium dioxide mole ratio of about 1.20 to 1.25, the fines yield isgreater than 90%.

In accordance with the practice of the present invention, the excessphosphoric acid used in the reaction mixture results in phosphoric acidremaining on the surface of the titanium pyrophosphate product. Thereare several ways to remove the excess phosphoric acid from the powdersurface to eliminate agglomeration. The phosphoric acid may be removedby vaporization wherein the fine powder is heated at temperatures inexcess of 700° C. to remove the excess phosphoric acid.

The excess phosphoric acid may also be removed by extraction from theinsoluble titanium pyrophosphate with a liquid, such as water. A waterwash will remove the excess phosphoric acid and permit the recovery ofthe titanium pyrophosphate. The phosphoric acid may then be recoveredfrom the weak acidic solution.

In a preferred method the titanium pyrophosphate is conveyed to anaqueous extractor/separator to remove the excess phosphoric acid. Theextractor/separator places the reaction product in water with theappropriate mixing and agitation to remove the excess phosphoric acid.The coarse particles of titanium pyrophosphate settle from the acidsolution and then are separated from the resulting slurry byconventional means such as decantation. Fine particles of titaniumpyrophosphate are recovered by filtration or centrifugation stepsperformed in conventional processes known within the art. The resultingtitanium pyrophosphate product is then dried to remove excess moisture.

The acidic nature of the reaction mixture used in the present inventiveprocess requires the use of corrosion resistant materials. The preferredmaterial of construction for the reaction vessel and the additionalprocessing equipment is 316L stainless steel. However, other corrosionresistant materials are suitable for practicing the method of thepresent invention.

The method of the present invention results in the formation of titaniumpyrophosphate predominately as a powder. The material has a particlesize less than one micron in diameter. Preferably the fine, dispersiblepowder has a particle size ranging from about 0.03 to 0.30 microns. Thesurface area of the resulting fine powder is generally near 20 m² \g.The surface area of the titanium pyrophosphate is slightly higher thanthe surface area for most pigments.

At the preferred phosphorus pentoxide to titanium dioxide mole ratio ofabout 1.20 to about 1.25, the composition of the resulting product is,by weight, about 20.27% titanium and about 28.69% phosphorus. Theresulting fine titanium pyrophosphate powder has a refractive index ofabout 1.88 and exhibits reflectance in the near UV region of 300 to 400nanometers. In contrast, titanium dioxide pigments do not exhibit anyreflectivity in the near UV region.

The dispersible titanium pyrophosphate powder resulting from the methodof the present invention is suitable for use as a UV-reflective pigment.A fine titanium pyrophosphate powder reflects ultraviolet radiation andthereby reduces the absorption of solar radiation by the structure orobject protected with the material. The titanium pyrophosphate alsoprovides heat stability and resistance to chemical attack, and istherefore well suited for various other applications.

In one such application, the titanium pyrophosphate of this inventionprovides a protective coating by means of dispersing the fine powder ina plastic material which can be employed as a coating on varioussubstrates to be protected from UV light. Alternatively, the bulk of apolymer may be mixed with the fine particle titanium pyrophosphate ofthe invention rather than relying on a layer with a loading of fineparticles of titanium pyrophosphate. Any suitable plastic materialcontaining the fine particle can be employed. Typical polymers include,without limitation, polyethylene, polypropylene, polystyrene,polyacrylate, polymethyl methacrylate, polyvinyl chloride, polyvinylalcohol, polycarbonate, polyacetals, polyesters, polyamides, andpolyimides. Also, the fine particle titanium pyrophosphate of thisinvention can be dispersed into mixtures of polymers such as, withoutlimitation, polypropylene with polyisobutylene, polypropylene withpolyethylene and mixtures of different types of the same polymer such ashigh density and low density polyethylene. In addition, any suitablecopolymer can be employed as the base or substrate into which the fineparticle titanium pyrophosphate of this invention can be dispersed.Typical, non-limiting examples of copolymers are copolymers ofmonoolefins and diolefins with each other or with other vinyl monomers,form example ethylene/propylene copolymers, linear low densitypolyethylene and mixtures thereof with low density polyethylene,polypropylene/but-1-ene copolymers, propylene/isobutylene copolymers,ethylene/but-1-ene copolymers, ethylene/hexene copolymers,ethylene/methylpentene copolymers, ethylene/heptene copolymers,ethylene/octene copolymers, propylene/butadiene copolymers,isobutylene/isoprene copolymers, ethylene/alkyl acrylate copolymers,ethylene/alkyl methacrylate copolymers, ethylene vinyl acetatecopolymers and their copolymers with carbon monoxide or ethylene/acrylicacid copolymers and their salts (ionomers) as well terpolymers ofethylene with proplyene and a diene such as hexadiene, dicyclopentadieneor ethylidene-norbornene; and mixtures of such copolymers with oneanother and with polymers mentioned above.

A wide range of loadings of the fine particle titanium pyrophosphate ofthis invention effects UV reflectance and thus protection of thesubstrate involoved. Typical loadings, by weight, range from 5% to about60% and most usually from about 20% to about 50%. Such loadings resultin reflectance of UV light in the range of from about 15% to about 35%,depending upon the thickness of the polymer layer containing thedispersed fine particles of titanium pyrophosphate of this invention.The titanium pyrophosphate of this invention may be employed togetherwith typical pigments and other colorants to provide a combination ofabsorption and reflectance of light. Most typically titanium dioxide maybe combined in the polymer with titanium pyrophosphate.

The following examples, which constitute the best mode presentlycontemplated by the inventor for practicing the invention, are presentedsolely for the purpose of further illustrating and disclosing thepresent invention, and are not to be construed as a limitation on theinvention. All percentages given in the examples are by weight unlessotherwise noted.

EXAMPLES

Three samples were prepared in order to demonstrate the improved finetitanium pyrophosphate powder yields resulting from the method of thepresent invention. Each sample included a reaction mixture of anhydroustitanium dioxide in the anatase form added to concentrated phosphoricacid in a sigma blade mixer having a one liter capacity. The phosphoruspentoxide to titanium dioxide mole ratio was varied for each of thethree samples. In the first sample, 672 grams of 115% phosphoric acidwere added to 315 grams of titanium dioxide to provide a sample having aphosphorus pentoxide to titanium dioxide mole ratio of about 1.00. Asecond sample was prepared having 712 grams of 115% phosphoric acid and315 grams of titanium dioxide to obtain a phosphorus pentoxide totitanium dioxide mole ratio of about 1.06. The third sample containedabout 820 grams of 115% phosphoric acid and 315 grams of titaniumdioxide to provide a sample having a phosphorus pentoxide to titaniumdioxide mole ratio of about 1.22.

The resulting slurries were heated in a vacuum oven at 120° C. todislodge entrapped air which potentially reduces the contact between thereactants. The samples were then placed in ceramic crucibles and heatedin a furnace to 450° C. for 60 minutes to form titanium pyrophosphate.The resulting product was then washed in a water solution under vigorousagitation to remove excess phosphoric acid. The slurry was then allowedto settle in order to classify the material and reject coarse particles.The coarse particles settled to the bottom of the container while themajority of the fine particles remained suspended. The top 80-90%, byvolume, of the slurry containing the fine particles was decanted andcollected. The collected slurry having the suspended fine particles wasthen stirred, settled, and decanted a second time to further ensure therejection of coarse particles. The decanted portion containing thesuspended titanium pyrophosphate was centrifuged in order to separatethe fine particles from the solution. The fine particles were then driedfor 3 hours at 150° C. to remove excess moisture from the particlesurface.

In all three samples, the resulting product was a dispersible, finepowder having a refractive index of about 1.88. The particle size of thefine powder was less than 1 micron in diameter. The reflectance andtransmittance properties were measured in a pigmented film of polyvinylalcohol ("PVA") which was supported on a glass microscope slide. Thepigments were mixed with a 7 weight percent solution of PVA in water.Weight ratios of pigment to dry PVA were either 0.5 or 1.0. The pigmentswere mixed in the solution first by a mortar and pestle and then in ahigh speed virtishear mixer. The resulting slurries were spread on cleanmicroscope slides using a coating rod to insure as much consistency infilm thickness as possible. The coated slides were allowed to dry 60° C.The water lost during drying resulted in coatings having a 33 or 50weight percent pigment content. The thickness of the coatings wasdetermined using a Sloan DEKTAK 3030 instrument. The coatings measuredfour to two microns in thickness for 33% or 50% pigment loadings,respectively. A 33% pigment loading exhibited a reflectance of about 25%at 350 nm whereas the 50% pigment loading exhibited a reflectance ofabout 30% at 350 nm. The total light transmission for the 33% and 50%pigment loadings were about 74.4% and 70.8%, respectively. Thereflectance and transmittance properties were recorded using aPerkin-Elmer Lambda 9 Spectrophotometer. The reflectance spectra wasobtained using an integrating sphere compartment as measured against aLab Sphere SRS-99 white reflectance standard.

The fines yields for the three samples were calculated by stirring0.10-0.15 grams of the resulting product, prior to separation of thecoarse material, in 25 ml of water at ambient temperature for one hourat 680 rpm with a stir bar in a 50 ml beaker. The slurry was allowed tosit for about three minutes before the top 20 ml was withdrawn. Theremaining 5 ml was filtered through a #1 Whatman filter paper undervacuum. The finer material passed through the filter. The coarse residuefrom the filter paper was then dried at 150° C. for one hour andweighed. The results for all three samples are listed in Table I.

The P₂ O₅ yield for the three samples was determined by extraction ofabout 0.25 grams in 25 ml of water for one hour at ambient temperature.The cloudy sample was centrifuged and 10.0 ml of the supernatant wasadded to 15 ml of water and 5 ml of concentrated hydrochloric acid. Thesample was boiled for one hour to convert all P₂ O₅ to theorthophosphate form. The sample was titrated with 0.1N sodium hydroxideto determine the amount of unreacted phosphoric acid. The P₂ O₅ yield isbased on a stoichiometric reaction between titanium dioxide andphosphorus pentoxide with titanium dioxide as the limiting reagent.

                  TABLE I                                                         ______________________________________                                              Mole                                                                             Ratio   H.sub.3 PO.sub.4  Calcination Calcination Fines P.sub.2                                                  O.sub.5                             Sample P.sub.2 O5: Conc.      Temp.       Time        Yield Yield                                                        No.    TiO.sub.2     (%)                                                         (° C.) (Min)                                                       (%)   (%)                         ______________________________________                                        1     1.00    115     450     60      52    --                                  2   1.06    115        450         60          74    95                       3   1.22    115        450         60          93    101                    ______________________________________                                    

It is to be understood that the forms of the invention herewith shownand described are to be taken as illustrative embodiments only of thesame, and that various changes in the shape, size and arrangement ofparts, as well as various procedural changes, may be resorted to withoutdeparting from the spirit of the invention.

What is claimed is:
 1. An article of manufacture having UV reflectancecomprising a polymer having dispersed therein a UV reflecting amount offine titanium pyrophosphate powder prepared by thermally reactingtitanium dioxide with phosphoric acid in a reaction mixture having aphosphorus pentoxide to titanium dioxide mole ratio greater than one andremoving excess phosphoric acid from said fine titanium pyrophosphatepowder.
 2. An article of claim 1 wherein the polymer is selected fromthe group consisting of polyethylene, polypropylene, polystyrene,polyacrylate, polymethyl methacrylate, polyvinyl chloride, polyvinylalcohol, polycarbonate, polyacetals, polyesters, polyamides, andpolyimides and mixtures thereof.
 3. An article of claim 2 wherein thepolymer is polyvinyl alcohol.
 4. An article of claim 1 wherein thepolymer contains from about 20% to about 60%, by weight, titaniumpyrophosphate.
 5. An article of claim 1 wherein the polymer is acopolymer selected from the group consisting of ethylene/propylenecopolymers, linear low density polyethylene and mixtures thereof withlow density polyethylene, polypropylene/but-1-ene copolymers,propylene/isobutylene copolymers, ethylene/but-1-ene copolymers,ethylene/hexene copolymers, ethylene/methylpentene copolymers,ethylene/heptene copolymers, ethylene/octene copolymers,propylene/butadiene copolymers, isobutylene/isoprene copolymers,ethylene/alkyl acrylate copolymers, ethylene/alkyl methacrylatecopolymers, ethylene vinyl acetate copolymers and their copolymers withcarbon monoxide or ethylene/acrylic acid copolymers and their salts(ionomers).
 6. An article of claim 1 wherein the polymer is aterpolymer.
 7. An article of claim 6 wherein the terpolymer is selectedfrom the group consisting of terpolymer from ethylene with propylene,hexadiene, dicyclopentadiene or ethylidenenorbomene; and mixtures ofsuch copolymers with one another and with polymers selected from thegroup consisting of ethylene/propylene copolymers, linear low densitypolyethylene and mixtures thereof with low density polyethylene,polypropylene/but-1-ene copolymers, propylene/isobutylene copolymers,ethylene/but-1-ene copolymers, ethylene/hexene copolymers,ethylene/methylpentene copolymers, ethylene/heptene copolymers,ethylene/octene copolymers, propylene/butadiene copolymers,isobutylene/isoprene copolymers, ethylene/alkyl acrylate copolymers,ethylene/alkyl methacrylate copolymers, ethylene vinyl acetatecopolymers and their copolymers with carbon monoxide or ethylene/acrylicacid copolymers and their salts (ionomers).